1. Technical Field
This invention relates generally to semiconductor memories.
2. Description of the Related Art
Phase change memory devices use phase change materials, i.e., materials that may be electrically switched between a generally amorphous and a generally crystalline state, as an electronic memory. One type of memory element utilizes a phase change material that may be electrically switched between generally amorphous and generally crystalline local orders or between different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline states.
Typical materials suitable for such an application include various chalcogenide elements. The state of the phase change materials is also non-volatile, absent application of excess temperatures, such as those in excess of 150° C. for extended times. When the memory is set in either a crystalline, semi-crystalline, amorphous, or semi-amorphous state representing a resistance value, that value is retained until reprogrammed, even if power is removed. This is because the programmed value represents a phase or physical state of the material (e.g., crystalline or amorphous).
The memory cells can be selected for a reading operation, for example, by applying suitable voltages to the respective word lines and suitable current pulses to the respective bit lines. A voltage reached at the bit line depends on the resistance of the storage element, i.e., the logic value stored in the selected memory cell.
The logic value stored in the memory cell is evaluated by using sense amplifiers to detect a difference in voltage and/or current reflecting the state of the memory. Typically, a sense amplifier includes a comparator receiving the bit line voltage, or a related voltage, and a suitable reference voltage for comparison to the level from the memory, after a period. For example, where the bit line voltage is higher than the reference voltage after a period of time, the memory state stored is described as a reset or logic value “0”, whereas in the case in which the bit line voltage is lower than the reference voltage, the stored logic value is described as set or a “1”.
The access element can consist of a threshold switch made of a phase-change material, similar to the storage element serially connected thereto. This element switches (potentially without changing its phase) from a high-resistance condition to a low-resistance condition when a voltage applied thereacross exceeds a threshold value and may revert to the high-resistance condition when a current flowing therethrough falls below a minimum holding value. While in the reset state, a transition from the high resistance to low resistance may entail a negative resistance range where, if the capacitance and transient current are low or of short duration, the phase state may be retained for some number of cycles without refreshing or rewriting the reset state. A voltage across the access element in the low-resistance condition has a substantially constant value (holding voltage) because the dynamic resistance dV/dI is relatively low so most of the voltage drop is a holding voltage, Vh, because the voltage is Vh+current×dV/dI. In this case, the matrix of memory cells can be advantageously provided without any transistor, and then using a single technological process.
During a reading operation, the read current pulse causes the charging of stray capacitance intrinsically associated with the bit line, and, accordingly, causes a corresponding transient of the bit line voltage on a selected bit line. If charged positively, the respective bit line voltage raises towards a corresponding steady value, depending on the logic value stored in the selected memory cell.
Generally, the write endurance or the number of write cycles that a phase change memory may withstand is lower for writing than reading. However, triggering a threshold device on during reading forces significant read and capacitive current through the threshold device and the memory element which may adversely affect lifetime and/or endurance.